What happened
On the morning of May 5, 2001, a Crossair Saab2000, registration HB-IYE, was operating a scheduled flight from Lugano-Agno to Bern-Belp. While following the MONIN 5M arrival procedure, the aircraft was instructed by Zurich Radar West to descend from flight level 160 to 110, with a specific requirement to expedite the descent through flight level 140. To comply with this instruction, the pilot performed a high-rate descent of approximately 3,000 feet per minute.
Simultaneously, a Swissair Airbus A321, registration HB-IOG, was flying from Geneva to Zurich at flight level 140. The air traffic controller instructed the Airbus to turn left to a heading of 020 degrees and provided traffic information regarding another aircraft crossing its altitude.
During the descent, the crew of the Saab2000 received both a Traffic Advisory (TCAS-TA) and a Resolution Advisory (TCAS-RA). The pilot responded to the RA by immediately increasing the descent rate to over 4,000 feet per minute, disconnecting the autopilot, and executing a slight right turn. The crew of the Airbus A321 received a TCAS-TA but no RA. The two aircraft passed extremely close to one another, with the Saab2000 passing approximately 100ft below the Airbus. The crew of the Airbus A320 managed to establish visual contact with the Saab2000 during the encounter.
The investigation
The investigation examined the separation management within the Zurich Radar West sector. It was established that the controller was managing the sector alone at the time of the incident, as the Radar Planning officer had briefly left the workstation. The investigation reviewed the flight paths of the two primary aircraft and a third aircraft, an Engiadina RQX600, which was also being managed in the same airspace.
Findings
- The primary cause of the incident was an inappropriate separation concept employed by the Radar Executive West controller.
- The controller's plan to manage the crossing of multiple aircraft was flawed; the time available for the Saab2000 to descend through flight level 140 was insufficient for a standard descent rate, necessitating an extremely high and risky descent rate of 4,000 feet per minute.
- The controller failed to intervene more forcefully or utilize alternative routing to prevent the conflict.
- The temporary absence of the Radar Planning officer may have contributed to the incident, although the controller stated he could have managed the traffic alone.